Erosion-resistant and corrosion-resistant white cast iron

Abstract

A hypereutectic white iron casting having, in the as-cast form of the casting, a microstructure comprising a ferrous matrix comprising 12-20% by weight of chromium dissolved in the matrix, eutectic chromium carbides dispersed in the matrix, primary chromium carbides dispersed in the matrix, and optionally secondary carbides dispersed in the matrix. The eutectic carbide is 15-25% by volume of the casting and the primary carbide is 25-35% by volume of the casting. The secondary carbide, when present, is at most 6% by volume of the casting.

Description

[0001] This application is a divisional application of the invention patent application with the application date of June 26, 2017, the international application number of PCT / AU2017 / 050650, the national application number of 201780039270.5, and the invention name of "Erosion and Corrosion Resistant White Cast Iron". Technical Field

[0002] The present invention relates to a scratch, impact, erosion and corrosion resistant white cast iron alloy comprising a hard material dispersed in a host metal or metal alloy.

[0003] The present invention also relates to equipment such as pump components (including components of slurry pumps) used in the mining and mineral processing industries, the equipment including castings of wear resistant material or white cast iron facings, wherein the equipment is exposed to any or more than one of severe abrasion, impact, erosion and corrosive wear.

[0004] The present invention also relates to a method of forming a white cast iron alloy.

[0005] The invention also relates to a method of forming a casting or cladding of white cast iron as at least a part of equipment used in the mining and mineral processing industries. Background Art

[0006] Equipment used in the mining and mineral processing industries is often subjected to severe wear from any one or more of abrasion, impact, erosion and corrosion.

[0007] Such equipment includes, for example, slurry pumps and lines, mill linings, crushers, transfer chutes and ground-engaging tools.

[0008] As a specific example, the metal "wet-end" components in a slurry pump are subject to scraping, impact, erosion and corrosive wear in use due to high tonnage of sharp and hard mineral particles passing through the pump. The pump components include frame liners, impellers, volutes and throat bushes. Typically, the specifications of the components range from 2 kilograms to about 20 or more tons by mass. The components include castings of wear-resistant materials or coverings of wear-resistant materials, wherein the equipment is subject to any or more than one of severe scraping, impact, erosion and corrosive wear and requires replacement at regular intervals to maintain pump performance in use.

[0009] Material loss in the metal wet-end components of a slurry pump while in service can be attributed to one or more of the following mechanisms:

[0010] • Erosive wear due to mineral particles (nominal 0.1-100 mm diameter) flowing through the equipment.

[0011] - Corrosion as a result of contact with liquid (this term encompasses ore slurries) flowing through the pump, where the pH of the liquid may vary from very acidic to very alkaline.

[0012] · Crushing or breaking due to impact load during use.

[0013] The family of high chromium white cast irons (HCWCI) described in ISO 21988, parts 1c) and 3.3, provides a range of alloys that optimize the three main properties required for slurry pump wet-end components over a wide range of operating conditions: (a) wear resistance, (b) corrosion resistance, and (c) fracture toughness.

[0014] The first HCWCI was developed 100 years ago and licensed in 1917 (US Patent 1,245,552).

[0015] The nominal bulk chemistry of the first HCWCI alloy is:

[0016] Chromium: 20-35% by weight.

[0017] Carbon: 1.5-3.0 wt%.

[0018] Silicon: 0.0-3.0% by weight.

[0019] Iron: Balance.

[0020] The first HCWCI alloy, designated "Cr27" in Table 3 of ISO 21988 and hereinafter referred to as "Cr27", complies with the claims of US Patent 1,245,552 and is essentially the "workhorse" material used today in many slurry pump applications subject to abrasive, erosive and corrosive wear.

[0021] The microstructure of Cr27 alloy castings consists of two significantly different phases, namely:

[0022] · 25 volume % chromium carbides.

[0023] · 75 volume % ferrous matrix.

[0024] The hardness of the chromium carbides in the microstructure (1400-1600 HV) is greater than the hardness of silica sand (900-1200 HV), the most common abrasive medium passing through slurry pumps, and these carbides impart excellent wear resistance to Cr27 castings.

[0025] The microscopic analysis of the chromium carbide and ferrous matrix phases of the Cr27 castings in the as-cast form of the castings (i.e., after the castings have been formed in the mold and continuously cooled to ambient temperature) and the bulk chemical composition of these phases are described in Table 1 listed below.

[0026] Table 1-Cr27 castings

[0027]

[0028] The following discussion of Cr27 castings is in the context of the castings in as-cast form.

[0029] The chemical composition of chromium carbides in Cr27 castings is Fe-62Cr-8.8C-2Mn and the stoichiometry is (Cr,Fe,Mn)7C3. The presence of hard chromium carbide phases in the microstructure of Cr27 castings imparts increased wear resistance to the castings.

[0030] The chemical composition of the ferrous matrix phase in the Cr27 casting is Fe-15Cr-0.8C-2Mn-0.5Si, which is essentially a martensitic stainless steel (hardness 600-800 HV) and provides good corrosion resistance in aqueous environments when pH>4.5.

[0031] The chromium carbides in the microstructure of the Cr27 castings comprise a three-dimensional continuous network which makes the Cr27 brittle and renders the castings vulnerable to shock loading conditions in use. As a result of the presence of the 3-D continuous network, the Cr27 castings have moderately low fracture toughness.

[0032] The liquidus temperature of Cr27 alloy is less than 1300°C and it is much easier to cast in a foundry than steels where the liquidus temperature is higher, typically around 1500°C.

[0033] The wear resistance of Cr27 castings is achieved by the presence of 25 volume % chromium carbides (CrC).

[0034] The corrosion resistance of the Cr27 casting is achieved by the presence of a 75 volume % stainless steel ferrous matrix containing 15 weight % dissolved elemental chromium.

[0035] Since the first HCWCI described above was developed about 100 years ago, there have been further developments in the field of high chromium white cast irons. These developments have led to improved properties in many areas.

[0036] As an example, the applicant developed a family of HCWCI grades designated Cr35 in order to manufacture slurry pump components for use in many high wear applications.

[0037] Cr35 was adopted by Standards Australia and International Standards as a designated wear resistant material and codified in AS / NZS2027 and ISO 21988 respectively about 10 years ago.

[0038] The wear resistance of the Cr35 family of alloys is recognized as being superior to that of Cr27 alloys in many slurry pump applications where erosive wear is the dominant mode of material loss.

[0039] The applicant has recognised that for some applications, including slurry pump applications (and for other equipment within a range of other applications) there remains a need for further improvements.

[0040] One specific area of ​​improvement is in slurry pump applications where the pH is < 4.5 due to the presence of acids and / or aeration and corrosion is the dominating factor in service life.

[0041] The above description should not be taken as common general knowledge in Australia or elsewhere. Summary of the invention

[0042] Applicants conducted an experimental program to establish factors that contribute to the performance of HCWCI slurry pump wet-end components in corrosive applications.

[0043] The purpose of the experimental program was to determine the optimum microstructure for HCWCI castings to achieve suitable performance in an environment where severe scratching, impact and erosion wear and high corrosion are present.

[0044] One outcome of the experimental program was the recognition that cast HCWCI slurry pump wet-end components having a specific microstructure can perform well in severe abrasion, impact, erosion and corrosion applications.

[0045] The microstructure of the present invention is defined in this specification in two states. One state is the microstructure of the casting in the as-cast form. The other state is the microstructure of the casting in the final use form.

[0046] Typically, the final use form of the casting is an as-cast casting that has been heat treated. Typically, the heat treatment increases the amount of chromium carbides in the matrix of the casting and reduces the amount of dissolved elemental chromium. Note that there are occasions where the final use form of the casting is an as-cast casting.

[0047] In summary, based on the results of the experimental program, the present invention provides a hypereutectic white cast iron casting having a microstructure in the as-cast form of the casting comprising: a ferrous matrix comprising 12-20 wt. % chromium dissolved in the matrix, eutectic chromium carbides dispersed in the matrix, primary chromium carbides dispersed in the matrix, and optionally secondary carbides dispersed in the matrix, wherein the eutectic carbides are 15-25 volume % of the casting, the primary carbides are 25-35 volume % of the casting, and the secondary carbides, when present, are up to 6 volume % of the casting.

[0048] The as-cast casting of the present invention described in the preceding paragraphs has a combination of the following features that provide suitable performance in applications where the components are exposed to severe scratching, impact and erosion wear and highly corrosive environments, such as for HCWCI slurry pump wet end components:

[0049] (a) high, at least 12 wt. % chromium dissolved in the matrix;

[0050] (b) a combination of eutectic carbides and primary chromium carbides dispersed in a matrix; and

[0051] (c) A high amount, typically at least 45 volume percent, of the combined eutectic carbides and primary chromium carbides.

[0052] The term "primary carbides" is understood to mean carbides which precipitate from the melt between the liquidus temperature and the solidus temperature.

[0053] The term "eutectic carbide" is understood to mean a carbide which precipitates from a melt at the solidus temperature.

[0054] The term "secondary carbides" is understood to mean carbides formed in the casting via solid state reactions.

[0055] The "as-cast form of the casting" mentioned in the preceding paragraph (and as used in the earlier part of the specification) is understood to mean the casting at the moment when the casting is formed in the mold and continuously cooled to ambient temperature. The cooling time may be a few minutes for smaller castings and several weeks for larger castings. Typically, the casting may be 1 or 2 kilograms and up to about 20 tons in mass.

[0056] The term "as-cast form of the casting" does not extend to castings that have been subjected to, for example, a post-casting heat treatment that results in the precipitation of secondary chromium carbides. An example of a secondary chromium carbide heat treatment comprises heating the casting to 950-1050°C and holding the casting at temperature for 4-6 hours and air cooling the casting to ambient temperature. The secondary chromium carbide heat treatment process causes Cr and C and other elements to precipitate from solution in the matrix and thereby changes the concentration of elements dissolved in the matrix. In the case of Cr, the reduction in elemental Cr dissolved in the matrix of the heat treated casting as a result of the secondary chromium carbide heat treatment process may be as high as 5% by weight, depending on the prior thermal history of the casting and the final heat treatment process.

[0057] Compared to the above microstructure of the as-cast casting, the heat treated as-cast casting may include (a) a lower concentration of dissolved chromium, (b) a smaller volume of the matrix; (c) the same concentration of primary and eutectic carbides, and (d) a higher volume of secondary carbides.

[0058] The concentration of dissolved chromium in the heat treated casting may be at least 12 weight percent.

[0059] The concentration of dissolved chromium in the heat treated casting may be at least 14 weight percent.

[0060] The concentration of dissolved chromium in the heat treated casting may be less than 20 weight percent.

[0061] Typically, the weight ratio of elemental chromium and carbon in the as-cast and heat-treated castings is selected to optimize the formation of "hard" carbides as eutectic carbides, primary carbides and secondary carbides in the as-cast and heat-treated castings.

[0062] The term "hard" is a relative term. In the context of the present invention, those skilled in the art have a clear understanding of what constitutes a hard carbide. For example, those skilled in the art understand that "hard" carbides include M7C3 carbides (where "M" includes Cr, Fe, and Mn). In contrast, M7C3 carbides are harder than M 23 C6 carbide is hard and M 23 C6 carbide is considered a "soft" carbide.

[0063] In this context, the applicant has recognised that as the chromium concentration increases in the hypereutectic white cast iron alloy of the present invention, ie in the bulk chemical composition of the alloy forming the casting, the carbides have the property of being M 23 The tendency for the softer phase of C6 carbides to transform / form instead of being the harder phase of M7C3 carbides.

[0064] Where optimum hardness is desired, it is preferred that the weight ratio of chromium to carbon in both the as-cast and heat-treated castings be greater than 7:1 and less than 9.25:1.

[0065] Typically, the ratio of chromium to carbon in both the as-cast and heat-treated castings is greater than 7.5:1.

[0066] The ratio of chromium to carbon in both as-cast and heat-treated castings may be greater than 8:1.

[0067] The eutectic carbides, primary carbides, and the secondary carbides in the as-cast casting and the heat-treated casting may be M7C3 carbides (wherein "M" includes Cr, Fe, and Mn).

[0068] The eutectic (Cr,Fe,Mn)7C3 carbides and primary (Cr,Fe,Mn)7C3 carbides in the as-cast castings and the heat-treated castings may each include: Cr: 50-70 wt. %, C: 8.5-8.9 wt. %, and Mn: 0.5-5.0 wt. %, and other elements, and the balance Fe.

[0069] The eutectic (Cr,Fe,Mn)7C3 carbides and primary (Cr,Fe,Mn)7C3 carbides in the as-cast castings and the heat-treated castings may each include: Cr: 55-65 wt. %, C: 8.5-8.9 wt. %, and Mn: 0.5-5.0 wt. %, and other elements, and the balance Fe.

[0070] The eutectic carbides in the as-cast and heat-treated castings may be fine-grained (grained) carbides, such as similar to the chromium carbides in Cr27 castings.

[0071] The primary carbides in the as-cast casting and the heat-treated casting may be coarse-grained carbides.

[0072] The secondary (Cr,Fe,Mn)7C3 carbides in the as-cast casting and the heat-treated casting may include: Cr: 45 wt %, C: 9 wt %, and Mn: 4 wt %, and other elements, and the balance Fe.

[0073] The secondary carbides in the as-cast casting and the heat-treated casting may be fine-grained carbides.

[0074] The ferrous matrix in the as-cast casting may include: Cr: 12-20 wt %, C: 0.2-1.5 wt % and Mn: 1.0-5.0 wt %, with the balance being Fe.

[0075] The ferrous matrix in the as-cast casting may include: Cr: 14-16 wt %, C: 0.3-1.2 wt % and Mn: 1.0-5.0 wt %, with the balance being Fe.

[0076] The ferrous matrix in the as-cast casting may include 13-17 wt. % Cr dissolved in the matrix.

[0077] The ferrous matrix in the as-cast casting may include 15 wt. % Cr dissolved in the matrix.

[0078] The as-cast casting may include 25-30 volume % primary carbides, 15-20 volume % eutectic carbides and up to 6 volume % secondary carbides.

[0079] Typically, the as-cast casting comprises 25-28 volume % primary carbides, 17-20 volume % eutectic carbides and up to 6 volume % secondary carbides.

[0080] The sum of eutectic carbides and primary chromium carbides in the as-cast casting may be greater than 45 volume %.

[0081] The sum of eutectic carbides and primary chromium carbides in the as-cast casting may be greater than 50 volume %.

[0082] The sum of eutectic carbides and primary chromium carbides in the as-cast casting may be less than 55 volume %.

[0083] The as-cast casting may include at least 2 volume % secondary carbides.

[0084] The ferrous matrix of the as-cast casting may be substantially martensite.

[0085] The ferrous matrix of the as-cast casting may consist of martensite with some retained austenite.

[0086] The ferrous matrix of the heat treated casting may consist of martensite.

[0087] The casting may be at least 1 tonne.

[0088] The casting may be at least 2 tons.

[0089] The casting may be at least 3 tons.

[0090] The casting may be produced by inoculation casting as described, for example, in Australian Patent 698777 in the name of the applicant and the disclosure of which is incorporated herein by cross reference.

[0091] The bulk chemical composition of the as-cast and heat-treated castings may be: 35-40 wt% Cr, 4-5 wt% C, <4 wt% Mn, <1.5% Si, and the balance Fe and impurities.

[0092] The bulk chemical composition may have a weight ratio of chromium to carbon greater than 7:1 and less than 9.25:1.

[0093] The bulk chemical composition may have a C concentration greater than 4.3 wt%.

[0094] The bulk chemical composition may have a C concentration of less than 4.7 wt %.

[0095] The bulk chemical composition may have a Mn concentration greater than 1 wt%.

[0096] The bulk chemical composition may have a Mn concentration of less than 3 wt%.

[0097] The bulk chemical composition may have a Si concentration greater than 0.5 wt %.

[0098] The bulk chemistry may have a Si concentration of less than 1 wt %.

[0099] Impurities may include sulfur, phosphorus, aluminum, nickel, copper and molybdenum.

[0100] In some cases, depending on foundry practices, the concentration of impurities can be quite high. For example, the Ni concentration can be as high as 2 wt % in some cases. Note that at such concentrations, Ni does not affect the hardness of the ferrous matrix because Ni is a strong austenite stabilizer and affects the phase transformation from austenite to martensite. However, because Ni cannot enter the chromium carbides and all of the Ni remains in the ferrous matrix, it has very little effect on the microstructure of the material at such concentrations. Preferably, the Ni concentration is less than 2.5 wt %.

[0101] The bulk chemistry of the as-cast and heat-treated castings may include positive additions of any one or more of the following compounds: carbides and / or nitrides and / or borides of niobium, titanium, tungsten, molybdenum, tantalum, vanadium and zirconium.

[0102] The wear resistance of the casting can be selected as required for the final use application of the casting. Wear resistance is not a property of the material. Wear resistance is a property of the system and depends on a number of operating factors, such as the hardness of the slurry particles, the size and angularity of the slurry particles, the slurry velocity, and the slurry pH in the case of a pump conveying a slurry.

[0103] Similarly, the corrosion resistance of the casting may be selected as desired for the end use application of the casting.Corrosion resistance is not a property of the material and, as is the case with wear resistance, depends on a number of operating factors.

[0104] The fracture toughness of the casting may be in the range of 20-40 MPa.m<1 / 2>, which is determined by the test procedure described in ASTM STP 559. The disclosure of ASTM STP 559 is incorporated herein by cross-reference.

[0105] The invention also includes equipment such as pump components used in the mining and mineral processing industries comprising the above-described castings, wherein the equipment is exposed to any or more than one of severe abrasive, erosive and corrosive wear.

[0106] The apparatus may include a casting in a heat-treated form, wherein as a result of the heat treatment, the microstructure has (a) a lower concentration of dissolved chromium, (b) a smaller volume of the matrix, (c) the same concentration of primary and eutectic carbides, and (d) a higher volume of secondary carbides.

[0107] The concentration of dissolved chromium in the heat treated casting may be at least 12 weight percent.

[0108] The concentration of dissolved chromium in the heat treated casting may be at least 14 weight percent.

[0109] The concentration of dissolved chromium in the heat treated casting may be less than 20 weight percent.

[0110] As noted above, the equipment of particular interest to the applicant is the "wet end" component of a grinding mill circuit slurry pump.

[0111] The equipment may also include, for example, pipelines, mill liners, crushers, transfer chutes, and grounding tools.

[0112] The present invention also provides a method for manufacturing the above casting, which comprises the following steps:

[0113] (a) forming a melt of a high chromium white cast iron alloy;

[0114] (b) pouring the molten alloy into a mold and forming a hypereutectic white cast iron casting having a microstructure comprising a ferrous matrix comprising 12-20 wt. % dissolved chromium, eutectic chromium carbides dispersed in the matrix, and primary chromium carbides dispersed in the matrix, and optionally secondary carbides dispersed in the matrix, wherein in the as-cast form of the casting, the eutectic carbides are 15-25 volume % of the casting, the primary carbides are 25-35 volume % of the casting, and the secondary carbides, when present, are up to 6 volume % of the casting.

[0115] The method may be, for example, the seed casting method described in Australian Patent 698777 in the name of the applicant.

[0116] The method may include a post-casting heat treatment step.

[0117] The heat treating step may include heating the casting to 950-1050°C and holding the casting at temperature for 4-6 hours and air cooling the casting to ambient temperature.

[0118] The present invention also includes a white cast iron alloy having the following bulk chemical composition: 35-40 wt% Cr, 4-5 wt% C, <4 wt% Mn, <1.5% Si, and the balance Fe and impurities.

[0119] The weight ratio of Cr to C may be greater than 7:1 and less than 9.25:1.

[0120] Typically, the ratio of Cr to C is greater than 7.5:1.

[0121] The ratio of Cr to C may be greater than 8:1.

[0122] The bulk chemical composition may have a C concentration greater than 4.3 wt%.

[0123] The bulk chemical composition may have a C concentration of less than 4.7 wt %.

[0124] The bulk chemical composition may have a Mn concentration greater than 1 wt%.

[0125] The bulk chemical composition may have a Mn concentration of less than 3 wt%.

[0126] The bulk chemical composition may have a Si concentration greater than 0.5 wt %.

[0127] The bulk chemistry may have a Si concentration of less than 1 wt %.

[0128] Impurities may include sulfur, phosphorus, aluminum, nickel, copper and molybdenum. BRIEF DESCRIPTION OF THE DRAWINGS

[0129] Embodiments of the present invention will now be described by way of example with reference to the following figures, in which:

[0130] Figure 1 Pie chart illustrating the phases of an alloy casting according to the present invention produced and analyzed during the above-described experimental procedures performed by applicant;

[0131] Figure 2 are representative SEM images of as-cast and heat-treated castings of samples according to the present invention; and

[0132] Figure 3 Representative SEM images of test castings from the same heat as the field trials of the as-cast castings according to the present invention. DETAILED DESCRIPTION

[0133] As noted above, an experimental program conducted by applicants found that HCWCI slurry pump wet end components made from an experimental alloy having, in the as-cast form of the casting, (a) a chromium carbide content of approximately 45% by volume, and (b) a ferrous matrix containing a chromium content of approximately 15% by weight dissolved in the matrix, performed well in severe abrasion, impact, erosion and corrosion applications.

[0134] On the basis of the experimental program described, the applicant has realized that an as-cast casting having a combination of the following features provides suitable performance as a HCWCI slurry pump wet end component exposed to an environment in which there is severe scratching, impact and erosion wear and high corrosion:

[0135] (a) high, at least 12 wt. % chromium dissolved in the matrix;

[0136] (b) a combination of eutectic carbides and primary chromium carbides dispersed in a matrix; and

[0137] (c) A high amount, typically at least 45 volume percent, of the combined eutectic carbides and primary chromium carbides.

[0138] Additionally, the applicant has realised that an as-cast casting having the following microstructure has an optimised combination of improved toughness, good corrosion resistance and excellent wear resistance for a range of applications including "wet end" components in mill circuit slurry pumps, pipelines, mill liners, crushers, transfer chutes and earth tools:

[0139] (a) the ferrous matrix contains 12-20 wt. % dissolved chromium,

[0140] (b) 15-25 volume % of the casting comprises eutectic chromium carbides dispersed in the matrix,

[0141] (c) 25-35 volume % of the casting comprises primary chromium carbides dispersed in the matrix, and

[0142] (d) Optionally, up to 6 volume % of the casting comprises secondary carbides dispersed in the matrix.

[0143] exist Figure 1 The microstructure of the experimental alloys in as-cast form (ie, before any downstream post-casting processing) is schematically illustrated in the pie chart of .

[0144] refer to Figure 1 , the microstructure comprises:

[0145] - A ferrous matrix consisting of martensite and some retained austenite and 15 wt% chromium dissolved in the matrix, wherein the ferrous matrix comprises 55 volume % of the casting.

[0146] A fine continuous 3-D network of eutectic chromium carbides, similar to the chromium carbides in a Cr27 (20 volume %) casting that make up 20 volume % of the casting. The presence of a continuous 3-D network of eutectic chromium carbides in the microstructure of a Cr27 casting significantly reduces fracture toughness. The eutectic carbides are M7C3 carbides (where "M" includes Cr, Fe, and Mn).

[0147] · Coarse discrete primary chromium carbides constituting 25 volume % of the casting, which form during solidification and also adversely affect the fracture toughness of the casting by reducing the amount of tougher ferrous matrix in the microstructure. The primary carbides are M7C3 carbides (where "M" includes Cr, Fe and Mn).

[0148] Optionally, fine discrete secondary carbides constituting up to 6 volume % of the casting, which form after solidification and also adversely affect the fracture toughness of the casting by (a) reducing the amount of tougher ferrous matrix in the microstructure and (b) destabilizing the austenite (which degrades to martensite) phase. The secondary carbides are M7C3 carbides (where "M" includes Cr, Fe and Mn).

[0149] Figure 2 Representative SEM images of as-cast and heat treated castings of samples according to the present invention. The images have been marked to show the distribution of primary and eutectic carbides in the ferrous matrix.

[0150] In a nominal Fe-Cr-C alloy, the microstructural and microanalytical characteristics of the stoichiometric (Cr,Fe,Mn)7C3 carbides, the volume % of primary carbides, the volume % of eutectic carbides, the carbide distribution, and the amounts of elemental chromium, iron, and carbon in (a) carbides and (b) ferrous matrix of castings of the alloy depend considerably on the partitioning behavior of the individual elements in the alloy during the solidification and cooling processes used to form the castings.

[0151] The factors that determine the partition coefficient of each element are complex and cannot be known exactly, and must be established by "trial and error".

[0152] In the experimental project, the applicant produced a number of Fe-Cr-C-2Mn-0.5Si alloys in the laboratory and determined the resulting microstructure and microanalysis of the phases by detailed examination using scanning electron microscopy, energy dispersive spectrometry, wavelength dispersive spectrometry and X-ray diffraction.

[0153] Based on the experimental data, the applicant was able to establish a method similar to (or close to) Figure 1An alloy having selected desired microstructural features for three phases in the casting is shown in , wherein particular attention is paid to achieving the following requirements: 15 weight % chromium dissolved in the matrix, a ferrous matrix constituting 55 volume % of the casting, and eutectics and primary carbides each constituting 20 and 25 volume % of the casting, respectively.

[0154] The nominal bulk chemical compositions of castings having the microstructural features described in the preceding paragraphs were determined by microanalysis and summing the proportions of the various phases. Typical nominal bulk chemical compositions of example castings according to the present invention having selected microstructural features are shown in Table 2 below.

[0155] Table 2 - Nominal chemical compositions of example castings with selected microstructural features

[0156]

[0157] Applicants follow these steps in the selection process:

[0158] • Selecting 25 volume % primary carbides for the casting fixes the eutectic carbides at about 20 volume % and the ferrous matrix at about 55 volume % of the casting.

[0159] The chemical composition of the ferrous matrix in the casting is selected to be Fe-15Cr-0.8C-2Mn-0.7Si to fix the bulk carbon content of the alloy.

[0160] • The bulk carbon content of the alloy establishes the solidification parameters (liquidus and solidus temperatures) of the alloy. The liquidus temperature in turn determines the final amount of primary carbides in the microstructure.

[0161] Where trial castings were made using the data in Table 2 as a starting point, the microstructural characteristics of the trial castings were quantified and compared to the expected characteristics in Table 2.

[0162] · Iteratively fine-tune the bulk chemistry of successive castings to establish Figure 1 And if Figure 2 The final bulk chemical composition of the desired microstructural features exemplified in .

[0163] Regarding the final dot point, determining the desired bulk chemistry for producing a sample with a ferrous matrix containing approximately 15 wt% chromium content dissolved in the matrix at ambient temperature requires an assessment of the chromium content prior to cooling to ambient temperature. Noting that direct measurement at temperature is not possible, the measurement was performed by solution treating the sample at 1200°C followed by water quenching to ambient temperature. This treatment retains the dissolved chromium and the maximum elemental chromium content achievable in the ferrous matrix in the as-cast condition can then be determined.

[0164] In addition to the experimental program described above, the applicant has also manufactured a number of castings according to the invention and tested these in field trials, some of which have been completed and evaluated.

[0165] The castings are made according to the applicant's standard process for high chromium white cast iron. The process is an inoculation process described in a patent family including US Patent 5803152. The disclosure of the US Patent is incorporated herein by cross-reference. The castings are made from heats (charges, heats) of 1-3 tons of the selected bulk chemistry. The pouring temperature is in the range of 1350-1450°C. The castings are allowed to cool naturally in their molds. Depending on the specific field test application, the castings are heat treated.

[0166] One of the field test series was conducted on the impeller and front guard assembly of the applicant's 150 MCU pump in a mill circuit operated by a mining company. The test was run for 1766 hours and the wear rate was evaluated and compared with the wear rate of high chromium white cast iron currently used in the same type of pump in the same mill circuit.

[0167] Another test series was conducted on the impeller, front guard, frame liner and volute assembly of a 350 MCU pump in a mill circuit operated by the applicant at another mining company. The test was run for 4100 hours and the wear rate was evaluated and compared with the wear rate of high chromium white cast iron currently used in the same type of pump in the same mill circuit.

[0168] The wet chemical analysis of the bulk chemical composition used to form the casting in one of the field trials is listed in Table 3 below.

[0169] Table 3 - Wet Chemical Analysis

[0170] element Cr C Mn Ni Si Fe weight% 37.5 4.4 2.0 1.7 0.43 Bal.

[0171] The analysis was performed on the inoculated sample, and therefore less than about 1 wt. % chromium and less than about 0.1 wt. % carbon would have been present in the original casting.

[0172] Figure 3 A representative SEM image of a test casting made from the bulk chemistry in Table 3 in the same heat as the field test. The image shows the distribution of primary and eutectic carbides in the ferrous matrix. By estimation, the test casting (and thus the field test casting) contains 18 volume % eutectic chromium carbides, 28 volume % primary carbides, 2-3 volume % secondary carbides and 12-16 weight % Cr dissolved in the matrix.

[0173] The wear rate in the field trials was found to be 0.3-0.4 mm / day. This is a 40% improvement over the high chromium white cast iron currently used in the same type of pump in the same mill circuit.

[0174] From a practical point of view, when the actual product is cast in a mold in a foundry, the impact of the cooling conditions on the microstructure of the casting and the extent to which chromium and other elements will precipitate from solution will have to be considered. In the context of chromium enrichment, different amounts of chromium will precipitate from solution when the casting in the mold cools to ambient temperature depending on the thermal profile of the mold and the size of the casting. This will have to be taken into account when determining the bulk chemistry required to generate a ferrous matrix containing a target chromium content of about 15% by weight (or another target concentration) dissolved in the matrix at ambient temperature.

[0175] Additionally, note that in standard foundry practice, the alloy castings may be subjected to a further heat treatment process, such as heating to 950-1050°C, holding at temperature for 4-6 hours, and air cooling to ambient temperature. This heat treatment process hardens the ferrous matrix by 100-200 Bain hardness points as follows:

[0176] (a) secondary hardening due to precipitation of secondary chromium carbides in the ferrous matrix and destabilization of retained austenite in the ferrous matrix; and

[0177] (b) Any Cr-depleted austenite in the ferrous matrix subsequently transforms to martensite on cooling to room temperature.

[0178] It is estimated that the formation of secondary chromium carbide precipitates during such a 950-1050°C heat treatment will reduce the dissolved elemental chromium content of the ferrous matrix by up to 3 wt%.

[0179] Many changes may be made to the embodiments of the present invention described in conjunction with the accompanying drawings without departing from the spirit and scope of the present invention.

[0180] In the following claims and in the foregoing description of the invention, unless the context requires otherwise due to explicit language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" are used in an open sense, i.e., specifying the presence of the features described in various embodiments of the invention but not excluding the presence or addition of further features.

Claims

1. A hypereutectic white iron casting having, in the as-cast form of the casting, a microstructure comprising: a ferrous matrix comprising 12-20% by weight of chromium dissolved therein, eutectic carbides, the eutectic carbides being dispersed in the ferrous matrix, primary carbides, said primary carbides being dispersed in said ferrous matrix, and Optionally, secondary carbides, said secondary carbides being dispersed in said ferrous matrix; in, The eutectic carbides are 15-25% by volume of the casting, the primary carbides are 25-35% by volume of the casting, and the secondary carbides, when present, are up to 6% by volume of the casting. wherein the eutectic carbide, the primary carbide and the secondary carbide are M7C3 carbides, wherein "M" includes Cr, Fe and Mn; wherein the weight ratio of chromium to carbon in the as-cast casting and the heat-treated casting is greater than 8:1 and less than 9.25:1; and The bulk chemical composition of the casting includes: 35-40 wt% Cr, 4-5 wt% C, 1-3 wt% Mn, <1.5 wt% Si, and the remainder Fe and impurities.

2. The casting defined in claim 1, wherein the eutectic (Cr,Fe,Mn)7C3 carbide and the primary (Cr,Fe,Mn)7C3 carbide each comprise: Cr: 50-70 wt %, C: 8.5-8.9 wt %, and Mn: 0.5-5.0 wt %.

3. The casting defined in claim 1, wherein the eutectic (Cr,Fe,Mn)7C3 carbide and the primary (Cr,Fe,Mn)7C3 carbide each comprise: Cr: 55-65 wt %, C: 8.5-8.9 wt %, and Mn: 0.5-5.0 wt %.

4. A casting as defined in any one of the preceding claims wherein the ferrous matrix comprises: Cr: 12-20 wt %, C: 0.2-1.5 wt %, and Mn: 1.0-5.0 wt %.

5. A casting as defined in any one of the preceding claims wherein the ferrous matrix comprises: Cr: 14-16 wt %, C: 0.3-1.2 wt %, and Mn: 1.0-5.0 wt %.

6. A casting as defined in any one of the preceding claims wherein the ferrous matrix comprises 13-17 wt. % Cr dissolved in the ferrous matrix.

7. A casting as defined in any one of the preceding claims wherein the ferrous matrix comprises 15 wt. % Cr dissolved in the ferrous matrix.

8. A casting as defined in any one of the preceding claims comprising 25-30 volume % primary carbides, 15-20 volume % eutectic carbides and up to 6 volume % secondary carbides.

9. A casting as defined in any one of the preceding claims comprising 25-28 volume % primary carbides, 17-20 volume % eutectic carbides and up to 6 volume % secondary carbides.

10. A casting as defined in any one of the preceding claims wherein the sum of eutectic carbides and primary carbides in the as-cast casting is greater than 45 volume percent.

11. A casting as defined in any one of the preceding claims wherein the sum of eutectic carbides and primary carbides in the as-cast casting is greater than 50 volume percent.

12. A casting as defined in any one of the preceding claims wherein the sum of eutectic carbides and primary carbides in the as-cast casting is greater than 35 volume percent.

13. A casting as defined in any one of the preceding claims wherein the sum of eutectic carbides and primary carbides in the as-cast casting is less than 55 volume percent.

14. A casting as defined in any one of the preceding claims wherein the ferrous matrix is ​​substantially martensite.

15. A casting as defined in any one of the preceding claims wherein the concentration of C in the bulk chemical composition of the casting is greater than 4.3 wt%.

16. A casting as defined in any one of the preceding claims wherein the concentration of C in the bulk chemical composition of the casting is less than 4.7 wt%.

17. A casting as defined in any one of the preceding claims wherein the concentration of Si in the bulk chemical composition of the casting is greater than 0.5 wt%.

18. A casting as defined in any one of the preceding claims wherein the concentration of Si in the bulk chemical composition of the casting is less than 1 wt%.

19. A hypereutectic white iron casting having, in the as-cast form of the casting, a microstructure comprising: a ferrous matrix comprising 12-20 wt. % chromium dissolved therein; eutectic carbides, the eutectic carbides being dispersed in the ferrous matrix; primary carbides dispersed in the ferrous matrix; and Optionally, secondary carbides, said secondary carbides being dispersed in said ferrous matrix; in, The eutectic carbides are 15-25% by volume of the casting, the primary carbides are 25-35% by volume of the casting, and the secondary carbides, when present, are up to 6% by volume of the casting. wherein the eutectic carbide, the primary carbide and the secondary carbide are M7C3 carbides, wherein "M" includes Cr, Fe and Mn; wherein the weight ratio of chromium to carbon in the as-cast casting and the heat-treated casting is greater than 8:1 and less than 9.25:1; The bulk chemical composition of the casting includes: 35-40 wt% Cr, 4.3-5 wt% C, 1-3 wt% Mn, <1.5 wt% Si, and the balance Fe and impurities; and Wherein, the sum of the eutectic carbides and the primary carbides in the as-cast casting is greater than 50 volume %.

20. A hypereutectic white iron casting having, in the as-cast form of the casting, a microstructure comprising: a ferrous matrix comprising 12-20 wt. % chromium dissolved therein; eutectic carbides, the eutectic carbides being dispersed in the ferrous matrix; primary carbides, the primary carbides being dispersed in the ferrous matrix; and Optionally, secondary carbides, said secondary carbides being dispersed in said ferrous matrix; in, The eutectic carbides are 15-25% by volume of the casting, the primary carbides are 25-35% by volume of the casting, and the secondary carbides, when present, are up to 6% by volume of the casting. wherein the eutectic carbide, the primary carbide and the secondary carbide are M7C3 carbides, wherein "M" includes Cr, Fe and Mn; wherein the weight ratio of chromium to carbon in the as-cast casting and the heat-treated casting is greater than 8:1 and less than 9.25:1; and The bulk chemical composition of the casting includes: 35-40 wt% Cr, 4-5 wt% C, 1-3 wt% Mn, <1.5 wt% Si, and the balance Fe and impurities; and Wherein, in the heat treated form of the casting, the chromium dissolved in the ferrous matrix accounts for 12-20 weight % of the weight of the casting.

21. Equipment such as pump assemblies for use in the mining and mineral processing industries, the equipment comprising a casting as defined in any one of the preceding claims, wherein the equipment is exposed to any or more than one of severe abrasive, impact, erosive and corrosive wear.

22. The apparatus defined in claim 21 wherein said casting is in a heat treated form, and wherein as a result of the heat treatment, said microstructure has (a) a lower concentration of dissolved chromium, (b) a smaller volume of said ferrous matrix, (c) the same concentration of said primary carbides and said eutectic carbides, and (c) a higher volume of said secondary carbides.

23. The apparatus defined in claim 22 wherein the concentration of dissolved chromium in the heat treated casting is at least 12 weight percent.

24. Apparatus as defined in claim 22 or claim 23 wherein the concentration of dissolved chromium in the heat treated casting is less than 20% by weight.

25. A method of manufacturing a casting as defined in any one of claims 1 to 20, comprising the steps of: (a) forming a melt of a high chromium white cast iron alloy; (b) pouring the molten alloy into a mold and forming a hypereutectic white cast iron casting having a microstructure comprising: a ferrous matrix comprising 12-20 wt. % dissolved chromium, eutectic carbides dispersed in the ferrous matrix, primary carbides dispersed in the ferrous matrix, and optionally secondary carbides dispersed in the ferrous matrix, wherein in the as-cast form of the piece, the eutectic carbides are 15-25 volume % of the casting, the primary carbides are 25-35 volume % of the casting, and the secondary carbides are up to 6 volume % of the casting.

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